Institute for Geology and Paleontology
University of Innsbruck
Rock Glaciers - An introduction with examples from the Austrian Alps
This brief introduction is based mainly on observations and investigations on rock glaciers in the Ötztal Alps and Stubai Alps, Tyrol, and the Schobergruppe, Hohe Tauern National Park, Carinthia. The most characteristic features are documented by a number of photographs and by some diagrams.
Rock glaciers are lobate to
tongue-shaped bodies of frozen debris with interstitial ice
cement, ice lenses, or a core of massive ice. Rock glaciers
belong to the most spectacular and most widespread
periglacial phenomenon on earth.
The shape of rock glaciers may be
- tongue-shaped (length : width ration > 1)
- lobate (length : width ration < 1) or
In the Austrian Alps most rock glaciers are up to a few hundred meters long, 50 - 200 meters wide and cover an area of up to 0.2 km². Reichenkar rock glacier in the western Stubai Alps, 1400 m long, is one of the largest rock glaciers.
The surface is characterized by a coarse grained debris layer ("active layer"),
which is underlain by frozen debris and/or ice.
The surface of most active rock glaciers is characterized by well developed longitudinal and transversal ridges and furrows.
The front slope of active rock glaciers is characterized by a steep gradient (40 - 45°), is bare of vegetation and composed of fresh, unweathered material.
In the root zone of some rock glaciers a spoon-shaped depression is developed, which formed by the melting of massive ice under the debris layer.
Temperatures in the debris layer
The heat budget of the debris layer is very complex and depends on several parameters, particularly by the weather conditions, the grain-size distribution and thickness of the debris layer, and the ground ice below.
In summer a rapid decrease in air temperature is observed from the surface to a depth of 150 cm. During clear and warm weather in summer daily temperature cycles are recorded to a depth of 100 - 150 cm, with a time lag of several hours (the minimum temperature occurs between 10 - 12 a.m., and maximum temperature occurs between 6 and 12 p.m).
Temperatures at the base of the winter snow cover (BTS)
As a thick snow cover acts as an isolating layer, air temperature at the base of the winter snow cover is mainly influenced by the heat flow of the underlying ground ice. Therefore, air temperatures at the base of the winter snow cover (BTS) do not show any daily temperature variations, and on active rock glaciers are generally below -3°C, whereas on permafrost-free ground BTS is significantly higher, mostly ranging between -0.3°C and -1°C.
Temperatures of the meltwater released from active rock glaciers are very low and astonishingly constant during the whole melt season, ranging between 0.4 and 0.9°C. This low water temperature indicates that the meltwater released at the rock glacier spring was in direct contact with ice when flowing through the rock glacier.
Discharge of active rock glaciers is characterized by strong seasonal and diurnal variations.
Water released from active rock glaciers is derived from snowmelt, melting of permafrost and/or glacier ice, atmospheric precipitation during summer thunderstorms and groundwater.
Meltwater flows through the rock glaciers on the surface of the permafrost ice (at the permafrost table), particularly along furrows on the upper portion of the rock glacier as channel flow and subordinately as sheet flow, englacial (in the permafrost layer) along moulins, and along subglacial channels at the base of the permafrost layer, particularly in the middle and lower portion of the rock glaciers.
Discharge is controlled by a number of parameters: the thickness of the winter snow cover, the size of rock glacier and drainage area, the presence of cirque glacier(s) in the drainage area, the thickness and grain size of the debris mantle/active layer, the groundwater supply and the weather conditions during the melt period, particularly summer thunderstorms, causing pronounced seasonal and diurnal variations.
Water derived from snowmelt and rainfall events is immediately released causing floods which represent the quickflow component of the hydrograph.
Fair weather periods with intense melting of snow and ice in the early afternoon are characterized by diurnal variations in discharge.
Electrical conductivity of the water is low during high discharges due to high amounts of meltwater derived from snow/icemelt and/or precipitation, and high during cold weather periods and in autumn when discharge is very low and consists mainly of groundwater.
Most rock glaciers are characterized by average surface velocities of a few tens of centimeters per year. In Austria highest velocities are recorded from Hochebenkar rock glacier (up to 5 m/year) and Reichenkar rock glacier (up to 2.5 m/year in the snout zone and up to 6.94 m/year on a highly active part in the steep intermediate zone in the period 1998 - 1999).
For further informations please contact Karl Krainer or Wolfram Mostler
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